In a third generation partnership project universal mobile telecommunications system (3GPP UMTS), the MAC layer in the user equipment (UE) and in the Radio Network Controller (RNC) is responsible for scheduling of data transmission in the uplink and downlink, respectively. Transport channels form the interface between the MAC layer and the physical layer. In the physical layer, a set of transport channels is combined to form a Coded Composite Transport Channel (CCTrCH).
A Transport Format Combination Set (TFCS) is defined for each CCTrCH. Each Transport Format Combination (TFC) defines a Transport Format (TF) for each transport channel of the CCTrCH. The TF defines the data rate of a transport channel by setting the Transmission Time Interval (TTI) (in ms), the Transport Block (TB) size (in bits) and the Transport Block Set (TBS) size (in number of blocks).
A TB is the basic unit exchanged between the MAC and physical layers. A TBS is defined as a set of TBs, which are exchanged between physical and MAC layers at the same time and using the same transport channel. The TTI is defined as the inter-arrival time of TBSs. A TTI is equal to the periodicity at which a TBS is transferred from the MAC to the physical layer and then by the physical layer onto the radio interface.
The MAC gets data from the radio link control (RLC) layer. The interface between the MAC layer and the RLC layer is formed by logical channels, or radio bearers (RB). Each transport channel can carry more than one RB. The RLC maintains a buffer for each RB; each buffer contains a set of RLC service data units (SDUs). Some RLC configurations (but not all) allow the segmentation of SDUs into protocol data units (PDUs), some allow the concatenation of SDUs to build PDUs and some allow the use of padding PDUs. In the MAC layer, a MAC header may be added to the PDU to form a TB.
The MAC layer selects the PDU sizes for a given TTI and requests these PDUs from the RLC layer. The RLC then segments and/or concatenates the SDUs in order to satisfy the MAC request. The MAC then builds the TBs and sends the TBs to the physical layer to be sent over the air in the next TTI.
At the UE side, in order to perform TFC selection, there are some standard requirements that must be followed by the UE. These requirements are summarized below.
It is desirable to provide a method for TFC selection, obviating the need for the MAC to have to determine the power needed by each TFC in each time slot.